1. Field of the Invention
The present invention relates to a scanning optical apparatus for use in digital copying machines, laser printers, laser plotters, laser facsimiles, laser photoengravers, and, more particularly, to a scanning optical apparatus having a plane-inclination correction function and capable of reducing the curvature of the field.
2. Description of the Related Art
There have been known a scanning optical apparatus capable of scanning a medium to be deflectively scanned by a variety of deflectors with light from a light source. The scanning optical apparatuses of the type described above have been used as the scanning optical systems of digital copying machines, laser printers, laser plotters, laser facsimiles, laser photoengravers, and the like.
In the conventional scanning optical apparatus described above, a fact is known that the pitch of the main scanning lines is caused to be nonuniform due to the inclination of the plane of a reflection surface at the time of the deflection if the reflection surface is used for deflecting a light beam. Therefore, it is necessary for the plane inclination to be corrected.
If the deflection angle in the scanning optical apparatus is arranged to be a large degree at the time of the deflection scanning, the curvature of field takes place by the image-forming lens system. Therefore, the field curvature taking place due to the image-forming lens system must be corrected in both the main and sub-scanning directions.
On the one hand, the scanning optical apparatuses have been usually arranged to use a rotary polygon mirror in the optical deflector thereof since a high speed scanning operation is required recently. In the case where the rotary polygon mirror is used as the optical deflector, the above-described plane inclination takes place in each of the reflection surfaces, causing the scanning positions on the surface to be scanned to become nonuniform.
Therefore, scanning optical apparatuses each of which is provided with a correction optical system capable of preventing the plane inclination taking place in the case where the rotary polygon mirror is used as the deflector has been disclosed in Japanese Patent Laying Open (KOKAI) No. 63-106618, and Japanese Patent Laying Open (KOKAI) No. 62-147421.
On the other hand, in order to correct the field curvature taking place in the scanning optical apparatus, there has been a technology relating to a post-objective type scanning optical apparatus which is, for example, adapted in such a manner that an image forming lens system (which comprises a spherical lens) is disposed in front of the deflector. Another technology disclosed relates to a post and pre-objective hybrid type scanning optical apparatus adapted in such a manner that the image forming lens system (which comprises a spherical lens) is disposed in front of or behind the deflector. A technology relating to the former type of the scanning optical apparatus has been disclosed in Japanese Patent Laying Open (KOKAI) No. 57-14820 in which the field curvatures in both the main and the sub-scanning directions are corrected by moving the light source in the direction of the optical axis in synchronization with the deflective scanning operation. A technology relating to the latter type of the scanning optical apparatus has been disclosed in Japanese Patent Laying Open (KOKAI) No. 58-57108 in which the field curvature is similarly corrected in both the main and the sub-scanning directions by moving the collimator lens and the converging lens in the direction of the optical axis in synchronization with the deflective scanning operation.
As an alternative to the above-described structures in which the light source or the lens is moved, it might be considered feasible to employ a construction adapted in such a manner that the field curvature is corrected by using an optical element having a variable focal length. The optical elements capable of varying their focal length for the purpose of conducting the correction of the field curvature have been disclosed in, for example, Japanese Patent Laying Open (KOKAI) No. 52-32348, Japanese Patent Laying Open (KOKAI) No. 54-99654, Japanese Patent Laying Open (KOKAI) No. 62-119520, Japanese Patent Laying Open (KOKAI) No. 62-129814, Japanese Patent Laying Open (KOKAI) No. 62-129816, Japanese Patent Laying Open (KOKAI) No. 62-151824, Japanese Patent Laying Open (KOKAI) No. 62-153933, Japanese Patent Laying Open (KOKAI) No. 62-157008, Japanese Patent Laying Open (KOKAI) No. 59-62821.
In the case where the image forming lens system of the scanning optical system having the above-described function capable of correcting the above-described field curvature is a spherical lens, the plane inclination cannot be easily corrected.
In the scanning optical system having the above-described function capable of correcting the plane inclination, lenses each having a different power (refracting power) are disposed in its optical path in both the main scanning and sub-scanning directions. Therefore, an astigmatic difference is involved, causing the field curvatures to usually become different between the main scanning direction and the sub-scanning direction.
Therefore, a problem arises in that the field curvature cannot be sufficiently corrected in both the main scanning and sub-scanning directions if the above-described mechanism for correcting the field curvature is applied to the correction scanning optical system which involves the plane inclination, the spherical lens disposed in the optical path or the light source being moved as described above in the application above. In the above-described disclosures, the operations such as sine wave vibrations conducted at the time of the correction of the field curvature have been described, but no specific descriptions have been made about the control of the movements of the light source or the lens or the control of the variation of the focal length of the optical element which is necessary to be conducted when the field curvature is corrected by the conventional method.
In the scanning optical system having the rotary polygon mirror therein, a synchronization control ranged from several hundred to several thousand Hz must be conducted. Therefore, a problem arises in that the movement of the lens or the light source cannot follow the deflective scanning operation due to a delay in the control in the case of the control of a usual opened loop. As a result, a satisfactory effect of correcting the field curvature cannot be obtained.
In a usual optical system capable of correcting the plane inclination in which no mechanism capable of mechanically correcting the field curvature is provided, an f.theta. lens or the like is employed in its optical system for the purpose of correcting the field curvature by the effect obtainable from the lens structure. Therefore, the field curvatures in both the main scanning and the sub-scanning directions, the f.theta. characteristic (magnifying power error, uniform scanning feasibility), spherical aberration, sine conditions must be taken into consideration as the factors at the design of the optical system. However, the above-described factors are in the trade-off relationship. Therefore, it is difficult to simultaneously and satisfactorily satisfy all of the design factors above.
Recently, since the scanning optical systems have been arranged to be high density systems, therefore, the field curvature must be further precisely corrected in the scanning optical apparatuses having the optical systems capable of correcting the plane scanning. As a result, the required level of the precision in the f.theta. lens and the like has been raised and the number of the lenses has been increased. Therefore, the assembling and the adjustment of the components are difficult to be conducted. In addition, the overall cost of the scanning optical apparatus becomes excessive due to increase in the number of the components.